1. Field of the Invention
The present invention relates to an apparatus for annealing a semiconductor material by means of irradiation with a laser beam.
2. Description of Related Art
Techniques for crystallizing amorphous silicon films by irradiating them with a laser beam have been known. Other techniques have been known wherein irradiation with a laser beam is performed to recover crystallinity of a silicon film which has been damaged as a result of implantation of impurity ions and to activate implanted impurity ions.
As a typical example of the latter kind of techniques, a technique has been known wherein regions which are to become a source and a drain of a thin film transistor are annealed by irradiating them with a laser beam after implanting impurity ions which are typically phosphorus or boron.
Such a process utilizing irradiation with a laser beam (generally referred to as “laser process”) is characterized in that it causes substantially no thermal damage to a substrate. This is because a method utilizing irradiation with a laser beam only instantaneously heat the irradiated surface and the effect of the heating is not extended to the substrate.
This feature of causing no thermal damage to a substrate is important in fabricating active matrix type liquid crystal displays which recently have an expanding range of application.
There are demands for use of glass substrates as substrates of active matrix type liquid crystal displays from the viewpoint of cost and needs for such displays with a larger surface area.
However, a glass substrate can not withstand a heating process at temperatures as high as 600° C. or more or 700° C. or more. One effective technique for avoiding this problem is to perform the crystallization of a silicon film and the annealing after implantation of impurity ions as described above utilizing irradiation with a laser beam.
According to a method utilizing irradiation with laser beams, even if a glass substrate is used, there is substantially no thermal damage to the glass substrate. It is therefore possible to fabricate a thin film transistor having a crystalline silicon film even with a glass substrate.
However, since the area of a laser beam is small, a laser process has problems including low efficiency in processing a large area and low homogeneity in processing a large area.